Combined withdrawal of an interface between two immiscible fluids (two liquids or a liquid and a gas) has recently been studied by authors such as E. O. Tuck and J. M. van den Broek ("A cusp-like free surface flow due to a submerged source or sink", J. Austral. Math. Soc. Ser. B., 25, 433-450, 1984); L. K. Forbes and G. C. Hocking ("Flow caused by a point sink in a fluid having a free surface", J. Austral. Math. Soc. Ser. B., 32, 231-249, 1990); and T. J. Singler and J. F. Geer Singler ("A hybrid perturbation-Galerkin solution to a problem in selective withdrawal", Phys. Fluids A, 5, 1156-1166, 1993). It is acknowledged to be a particular case of a more general interfacial instability phenomenon known as selective withdrawal/combined withdrawal. Studies in this field have focused largely on the determination of parameters (e.g. the distance from the sink to the free surface, the fluid density ratio, the surface tension between the fluids) at the onset of combined withdrawal (i.e. of sweeping of the fluid behind the free surface when the fluid in front of it is withdrawn at a given distance from the surface). However, the fluid dynamics of the microjet produced by combined withdrawal seemingly remains unexplored. The observation and study of the microjet, its peculiar properties and its potential uses, led to the present atomization procedure.
Existing atomization methods convert the type of energy supplied to the system (e.g. kinetic energy of the gas in pneumatic atomizers, electrical energy in sonic and ultrasonic piezoelectric atomizers, mechanical energy in rotary atomizers, electrostatic energy in electrohydrodynamic atomizers, etc.) into surface tension free energy since the liquid-gas surface is dramatically expanded by the effect of these processes. As a function of the resulting degree of disorder, part of the energy is also degraded in the statistical dispersion of the resulting drop sizes. Depending on how disorderly and rapidly (or gradually and efficiently) the processes by which the above-mentioned energies are converted into free surface energy take place, the resulting sprays are suitable for different specific uses.
As a rule, the spray should consist of small drops of uniform size. A small drop size is always in conflict with a high flow-rate in the fluid to be atomized, which results in high energy use per time unit. Also, uniformity in drop size relies on gradual, non-turbulent, scarcely random processes that are incompatible with the rapid conversion of volumetric energy into surface energy required by the typically high liquid flow-rates involved in many cases and with technological simplicity in the atomizer. Mechanical simplicity and expeditiousness in the atomizer, and irreversibility and randomness in the atomization process, are all highly correlated.
Existing pneumatic atomizers involve the cascading breaking of the interface from a high Weber number to a unity Weber number, the latter being accomplished when drop diameters are such that surface tension forces offset the inertia of the gas relative to the liquid. Such atomizers include the straightforward coaxial model of S. Nukiyama and Y. Tanasawa ("Experiments on the atomization of liquids in the airstream", Trans. Soc. Mech. Eng. Jpn., 5, 68-75, 1939) or the airblast models developed by I. D. Wigg ("Drop-size predictions for twin fluid atomizers," J. Inst. Fuel, 27, 500-505, 1964), G. E. Lorenzetto and A. H. Lefebvre ("Measurements of drop size on a plain jet airblast atomizer", AIAA J., 15, 1006-1010, 1977), A. K. Jasuja ("Plain-jet airblast atomization of alternative liquid petroleum fuels under high ambient air pressure conditions", ASME Paper 82-GT-32. 1982), and N. K. Risk and A. H. Lefebvre ("Spray characteristics of plain-jet-airblast atomizers", Trans. ASME J. Eng. Gas Turbines Power, 106, 639-644, 1983), among many others, or that reported by A. Unal ("Flow separation and liquid rundown in gas-atomization process", Metall. Trans. B., 20B, 613-622, 1989), based on the coaxial atomization of a liquid metal using a supersonic gas flow.
Cascading processes in existing pneumatic atomizers involved highly turbulent flows and randomness, which result in highly disperse drop size and atomizates.
One other major disadvantage of this type of atomizer is the limited sizes it provides (above 20 microns on average at best).
Whistling atomizers also have their pitfalls, prominent among which are noise, a relative complexity--they use wave generators and piezoelectric devices to excite the capillary jet produced--, and a limited drop size (usually larger than 50 .mu.m).
One novel atomization system that also provides extremely small, monodisperse drop sizes is electrostatic or electrospray atomization. The system has been disclosed (e.g. by M. L. Colelough and T. J. Noakes, "Apparatus and process for producing powders and other granular materials", European Patent Application 87305187.4, 1987). The chief disadvantage of this method in many cases is that it requires using a high-voltage dc source--which poses serious problems--and hence a discharge system (e.g. electrical crowns), both of which add up to the inherent complexity, large weight and low manipulability of this system.